1. Field of the Invention
The present invention relates to a radio communication apparatus such as a receiver for compensating for distortion caused by multipath fading and correcting an error correction and a transmitter for transmission data to the receiver.
2. Description of the Related Art
In the field of the radio communication, it is indispensable for overcoming the problem of multipath fading and improving transmission quality. It is known that equalizers are useful for overcoming the problem of multipath fading. In order to improve transmission quality, there is used a method in which an error correction code, particularly a convolutional code is decoded by a Viterbi decoder for a soft decision.
A conventional data transmitter comprises an equalizer for compensating for distortion caused by multipath fading and a Viterbi decoder for an error correction in a receiving system, and the equalizer and the Viterbi decoder are independently operated.
FIG. 1 is a structural showing the conventional data transmitter in which the equalizer and the Viterbi decoder are independently operated. On the transmitter side, transmission data 1 is encoded by a convolution encoder 2, modulated by a modulator 3, and transmitted from a transmission antenna 4. On the receiver side, a received signal received by a receiving antenna 5 is sent to an equalizer 6, and an output of the equalizer 6 is decoded by a Viterbi decoder 7 so as to obtain decoded data 8. The equalizer 6 compensates for distortion caused by multipath fading occurring on a line, and an MLSE (Maximum Likelihood Sequence Estimator) or a DFE (Decision Feedback Equalizer) are used as such an equalizer. In particular, MLSE is known as an apparatus, which can realize substantially an optimum characteristic even in a mobile radio channel whose fading varies drastically
The following will explain the principle of the MLSE-typed equalizer 6 using a line model shown in FIG. 2.
The line model of FIG. 2 is a model of the multipath fading with paths of (N+1) waves. In this line model, a transmission signal 100 is delayed by delay units 101-0 to 101-N, and is subjected to fading variation by relay fading adding units 102-0 to 102-N. Then, attenuators 103-0 to 103-N attenuate the signal subjected to the fading variation, and the attenuated resultant is added by a complex adder 104 so as to obtain a received signal 105.
The delay units 101-0 to 101-N show the delay caused by various lengths of paths, and the relay fading adding units 102-0 to 102-N show the relay fading, which is independently provided to the respective paths. The transmission signal 100 is subjected to a random phase variation and a level variation in accordance with a relay distribution by these delay units and the relay fading adding units.
The attenuators 103-0 to 103-N show attenuation, which is independently provided to the respective paths. In a base band, the transmitting and receiving signals comprise a quadrature component and an in-phase component, which are respectively a real part and an imaginary part, and they are considered as complex numbers, and each section of FIG. 2 is a complex number. In other words, a model in which the signals are finally combined at the receiving antenna terminal is also used as a complex adder 104.
FIG. 3 shows a case in which the line model shown in FIG. 2 is rewritten as a model, which is close to a digital filter. A transmission signal 200 is delayed by delay units 202-0 to 201-(Nxe2x88x921), complex gains are added thereto by complex gain adding units 202-0 to 202-N, and combined by a complex adder 203 so as to obtain a received signal 204. The complex gain adding units 202-0 to 202-N are those in which the variations of the relay fading adding units 102-0 to 102-N and those of attenuators 103-0 to 103-N are combined.
The MLSE estimates the complex gain adding units 202-0 to 202-N using an unique word inserted to data. If the complex gain adding units 202-0 to 202-N are obtained, the line model can be reproduced. Therefore, in a state in which past transmission data 200 is stored by delay units 201-0 to 201-(Nxe2x88x921), a replica is generated using the filter of FIG. 3 and a transmitting sequence is estimated by Viterbi decoding.
However, there is a case in which the error rate can be reduced to only a certain degree in the MLSE. Due to this, an error correction code is used in conjunction with the MLSE. The convolution encoder 2 produces a plurality of bits in accordance with the state of the past several bits every time when one bit of transmission data 1 is input. For example, if an encoding rate is 1/2, two bits are produced every time one bit of transmission data 1 is input. This state is shown in FIG. 4. Transmission data 300 is delayed by delay units 301-0 to 301-(Mxe2x88x921), complex gains are added thereto by complex gain adding units 302-0 to 302-(M), and its exclusive-OR is obtained by an exclusive-OR circuit 303, generating a transmission signal 304. The past transmission data 300 is accumulated by the delay units 301-0 to 301-(Mxe2x88x921), and this is the state at the time of decoding by Viterbi decode. In the case of the convolution encoder, the encoder is unchanged. Moreover, the complex gain adding units 302-0 to 302-M actually obtain only any one of values of 0,1,j, 1+j since only a bit calculation is performed. In this case, since the structure of the encoder of FIG. 4 is shown in advance, in the Viterbi decoder 7 on the receiving side, data can be decoded by the Viterbi decoding.
Thus, in the conventional data transmitter, the equalizer such as MLSE compensates for the distortion of the multipath fading, and uncompensated errors are corrected by the error correction codes such the convolution encoding, Viterbi decoding, thereby realizing the data transmission with a good quality.
However, in the conventional data transmitter, the compensation for the line distortion using the equalizer and the error correction using the Viterbi decoder were independently performed. As a result, constraint conditions of the respective sequences were independent of each other. Among paths for Viterbi decoding, there was a case in which the path, which was impossible to be used as a candidate, was included, depending on the line condition. This resulted in the deterioration of improving effect of the error rate.
An object of the present invention is to provide a radio communication apparatus wherein equalization for removing a line distortion caused by multipath fading and an error correction for reducing an error rate are simultaneously performed so as to improve a receiving quality, and two steps, i.e., the equalization and the error correction are performed at one time so as to improve reduction in the number of times of trace back and a memory capacity.
According to the present invention, there is estimated an imaginary encoder in which a line model and a convolutional encoder are combined, thereby performing Viterbi decoding thereby, and the above-mentioned object can be attained by the radio communication apparatus, which can simultaneously perform equalization using an MLSE and Viterbi decoding of a convolutional code. Thereby, an error rate characteristic can be improved.